In conventional wireless packet communication apparatus, only one radio channel to be used is determined in advance, whether this radio channel is idle or not is detected (carrier sense) before transmission of a data packet, and one data packet is transmitted only if the radio channel is idle. This kind of control allows plural STAs to share a single radio channel by using it during periods that are deviated from each other ((1) International Standard ISO/IEC 8802-11 ANSI/EEE Std. 802.11, 1999 edition, Information technology—Telecommunications and information exchange between systems—local and metropolitan area networks—Specific requirements—part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications; (2) Low-powered Data Communication System/Broadband Mobile Access Communication System (CSMA) standard, ARIB STD-T71 version 1.0, Association of Radio Industries and Businesses, settled in 2000).
In such wireless packet communication apparatus, one method for increasing the maximum throughput is to increase the data transmission rate of a PHY layer by widening the frequency band per radio channel.
However, as pointed out in a document (Iizuka et al., “5 GHz Wireless LAN System based on the IEEE802.11a standard—Packet Transmission Characteristics—”, B-5-124, Proceedings of The Institute of Electronics, Information and Communication Engineers Society Conference 2000, September 2000), to avoid packet collision, it is necessary to set, immediately after transmission of a packet, a prescribed transmission deferral duration that is independent of the data transmission rate of a PHY layer. Where such a transmission deferral duration is set, the data packet transmission efficiency (i.e., the ratio of the maximum throughput to the data transmission rate of a PHY layer) decreases as the data transmission rate of a PHY layer increases. Therefore, it is difficult to increase the throughput greatly merely by increasing the data transmission rate of a PHY layer.
In contrast, application of a MIMO technique (Kurosaki et al., “100 Mbit/s SDM-COFDM over MIMO Channel for Broadband Mobile Communications,” Technical Reports of The Institute of Electronics, Information and Communication Engineers, A•P2001-96, RCS2001-135 (2001-10)) is being considered as a method for increasing the maximum throughput without increasing the frequency band per radio channel. This MIMO technique is such that different data packets are sent simultaneously from plural antennas on the same radio channel and the plural data packets transmitted simultaneously on the same radio channel are received by performing pieces of digital signal processing corresponding to respective propagation coefficient differences of data packets received by plural antennas of a cooperating STA. The MIMO number is determined in accordance with the propagation coefficient etc.
On the other hand, where each STA has plural wireless network interfaces and hence can use plural radio channels, it is expected that the throughput can be made higher than in the case of a communication method in which a single radio channel is time-divided, by causing plural STAs to use different radio channels.
However, where the center frequencies of the plural radio channels used simultaneously are close to each other, the influence of leakage power that leaks from the frequency range of one radio channel to that of another radio channel is large. In general, in transmitting a data packet, after a transmit-side station has sent a data packet a receive-side station returns, to the transmit-side station, a delivery acknowledgment packet (ACK packet, NACK packet) for a data packet received by the receive-side station. When the transmit-side station attempts to receive this delivery acknowledgment packet, leakage power from another radio channel that is being used simultaneously for transmission is problematic.
For example, assume a case that as shown in FIG. 21 the center frequencies of radio channels #1 and #2 are close to each other and different times are required for transmission of data packets that are transmitted simultaneously on those radio channels. In this example, the data packet that is transmitted on radio channel #1 is short. Therefore, radio channel #2 is still used for transmission when an ACK packet is received. As a result, the ACK packet may not be received on radio channel #1 due to leakage power from radio channel #2. In this type of situation, increase in throughput is not expected even if transmissions are performed simultaneously by using plural radio channels.
This kind of case occurs due to a difference between in packet time length ((time required for transmission)=(data size)) between data packets in the case where the transmission rates of respective radio channels are the same, and due to a difference in packet time length ((time required for transmission)=(data size)/(transmission rate)) between data packets in the case where the transmission rates of respective radio channels are taken into consideration.
Incidentally, in wireless LAN systems etc., the data sizes of data frames that are input from a network are not constant. Therefore, when input data frames are converted to data packets and transmitted sequentially, the packet time lengths (times required for transmission) of respective data packets vary. Therefore, as shown in FIG. 21, even if plural data packets are transmitted simultaneously, the packet time lengths of the respective data packets are different from each other and it is highly probable that the reception of an ACK packet fails.
An object of the present invention is to provide a wireless packet communication method and a wireless packet communication apparatus capable of transmitting plural data packets simultaneously between two STAs and increasing the throughput even if power leakage occurs between radio channels, in the case where each STA can use plural radio channels simultaneously.